This invention relates to electronic sub-assemblies, and particularly to structures comprising a printed wiring board laminate that is electrically and mechanically connected through an interposer connector to an area array device, such as another board, a module or other active or passive device.
The mechanical and electrical core of a computer system comprises a plurality of printed wiring boards interconnected with other boards or modules and with other active or passive devices, such as diodes, semiconductors, capacitors and resistors. The success or failure of such computer systems is dependent upon, among other factors, their ability to operate without mechanical breakdowns and electrical failures.
One type of computer system is a microprocessor called a network server. A principal function of a network server is to achieve organized channels of communication between a plurality of personal computers. It also serves to house many programs that it shares with the personal computers. Thus, the reliable operation of the server is critical to the operation of an entire network of such personal computers.
The interconnection between the mating surfaces of a printed wiring board and a module or other active or passive device may be provided through an interposer. An interposer is a structure that provides electrical contact between two such devices (hereinafter referred to as area array devices). These area array devices typically are approximately parallel. The electrical connection between an area array device and a printed wiring board is achieved by pressing the device and the wiring board together with the thin conductive interposer between the two. The interposer can include compressible conductive elastomers, coil springs or leaf springs to establish the electrical connection between the wiring board and the other device.
The need for area array connectors is growing due to the increase in inputs and outputs on area array modules. An area array connector is a type of high-density, low inductance socket available from a number of suppliers. These connectors may be any one of a number of different types. One such socket uses compressible xe2x80x98fuzz buttonsxe2x80x99. These are compressible wadded wires described, for example, in the following patents: U.S. Pat. Nos. 5,552,752; 5,146,453 and 5,631,446. These are small, irregularly wound and inter-twined pads or balls that are made of gold plated beryllium copper wool or gold plated molybdenum wire. These wadded wire balls are compressed in holes in the interposer, which is a thin sheet of insulative material that separates the printed wiring board from the area array device. The holes are arranged in a pattern that matches a pattern of conductive pads on the surfaces of the printed wiring board and the area array device in contact with the interposer. In the assembly process, the xe2x80x98fuzz buttonsxe2x80x99 are compressed between the conductive pads, thereby providing electrical connections between the pairs of pads on the two surfaces that are separated by the interposer. Among the other types of connectors are metal filled elastomers, such as those sold by Tyco Inc. (formerly Thomas and Betts) as Metal Particle Interconnect Elastomers. Metal springs are also used. These metal springs generally are leaf springs having a number of geometries, such as C-shaped or V-shaped. Soldered connections complicate the disassembly or separation of the printed wiring board from the area array modules if repair or replacement is required.
A limitation of this type of interconnect technology is the inherent failure rate that can be obtained while placing electrical contacts in series. The intrinsic failure rate of electrical contacts in series can be approximated by:
Failure rate=nxc2x7IFR,
wherein
Failure Rate equals the contacts in series
n=number of contacts in series
IFR=intrinsic failure of each contact interface.
Therefore, addition of more electrical contacts in series typically increases the failure rate of an electrical circuit.
Land grid area array interposers have been described in which an interposer is designed for electrically connecting an electrical device to a printed circuit board wherein the interposer has been fabricated to provide adequate stand-off between the electrical device and the printed circuit board to provide space for active and/or passive devices.
To provide a connection between a printed wiring board and another area array device (which may be a second printed wiring board) using a land grid array (LGA) connector, a space of about 3 mm is needed to allow clearance for any active or passive devices, such as diodes, capacitors and capacitors that are mounted on the surface of the board. An option is to request the vendors of the connector to devise a custom connector that provides the needed height. As is well recognized, the customization of any piece of hardware or software can add appreciably to the overall product development schedule, as well as the cost of assembly and/or use.
The present invention relates to an electronic sub-assembly and its method of manufacture. The sub-assembly comprises a printed wiring board (PWB) having a first surface including a plurality of vias terminating at electrical contacts at said surface. These contacts generally are in the form of contact pads. A first area array device having a surface, including a plurality of electrical contacts, is spaced from the PWB. A plurality of connectors electrically couple the electrical contacts on the surface of the PWB to the electrical contacts on the surface of the first area array device. The printed wiring board typically has a second surface that includes a plurality of electrical contacts that are coupled to the second surface of the PWB by a plurality of connectors. The vias extend through the PWB to form electrically conductive plated through holes.
In one embodiment, an interposer separates the first surface of the printed wiring board from the surface of the first area array device. A second interposer may also separate the second surface of the PWB from the second area array device. Each of the interposers includes the connectors that couple said printed circuit board to the respective area array device. The area array devices may be spaced a fixed minimum distance from the printed wiring board, preferably a distance of about 3 mm. The sub-assembly preferably uses connectors that are selected from the group consisting of compressible wadded wire contacts, metal springs, filled conductive elastomers, solder balls and hard solder balls.
For ease of repair and replacement, the connectors are compressible wadded wire having a first end soldered to said contact pads and a second end compressed against the electrical contacts on the surface of the area array device. This allows the area array device to be easily detached from the sub-assembly.
Means are also provided for maintaining a fixed minimum spacing between the PWB and each area array device
The invention also relates to a printed wiring board sub-assembly. The printed wiring board has a first surface containing one or more vias extending from the first surface into the board, and a conductive contact pad on the surface of the printed wiring board electrically joined to each of said vias. An interposer structure has a first surface facing the first surface of the printed wiring board, and a second surface. The structure includes a pattern of holes extending therethrough that correspond to the holes in the printed wiring board. Wadded wire electrical contacts are pressed into each hole in the pattern. Each of the electrical contacts has a first end that is soldered or is pasted to a contact pad on the first surface of the printed wiring board. The second end of the wadded wire contact is electrically coupled by compressive contact to a conductive lead on a surface of a module or another printed wiring board.
At least some of the vias typically extend through the board to form plated through holes that terminate at a second board surface. A second interposer structure can be joined to contact pads on the second surface in a manner similar to that of the first interposer to provide an interconnect to a second area array device.
In yet another embodiment, the invention relates to an electronic sub-assembly that comprises a PWB spaced a fixed minimum distance from an area array device, and the method of making the same. The printed wiring board has a first surface including a plurality of vias extending from the surface into the board. A plurality of electrical contact pads are connected to each of the vias, at least some of the contact pads being offset from the corresponding vias. The area array device has a surface including a plurality of electrical contacts. The surface of the PWB and the surface of the area array device are spaced a fixed minimum distance, preferably about 3 mm, from one another. A plurality of connectors electrically couple the contact pads on the surface of the printed circuit board to the electrical contacts on the surface of the area array device. The connectors are selected from the group consisting of compressible wadded wire contacts, metal springs, filled conductive elastomers, solder balls and hard solder balls. The hard solder balls typically are composed of copper balls coated with a layer containing tin. If normal solder balls are used, they may be embedded in an underfill material filling the space between the printed wiring board and the area array device. The wadded wire connectors, filled elastomers and metal springs typically are used by placing them in holes extending through the interposer between the surface of the PWB and the corresponding surface of the area array device. The interposer may also comprise a pair of plastic caps, each cap including a lip extending over an edge of the PWB.
In yet another embodiment, an electronic sub-assembly comprises a printed wiring board having first and second generally planar surfaces and a plurality of vias terminating in electrical contact pads on the surfaces. A first area array device has a generally planar surface facing the first generally planar surface of the printed wiring board. The surface of the array device has a plurality of electrical contacts thereon. A plurality of connectors electrically couple the electrical contacts on the first surface of the printed circuit board to the electrical contacts on the surface of the first area array device. These connectors can be wadded wire contacts, metal springs, filled conductive elastomers, solder balls or hard balls or combinations thereof. A first interposer separates the first surface of the printed wiring board from the surface of the first area array device a fixed minimum distance of about 3 mm. The interposer contains a pattern of holes corresponding in location to the contacts on the first surface of the printed wiring board and the connectors are positioned in these holes. The sub-assembly further includes a second area array device having a surface including a plurality of electrical contacts for electrically coupling the second surface of the printed circuit board to the second area array device. A second interposer separates the second surface of the printed wiring board from the surface of the second area array device. As with the first interposer, the second interposer contains a pattern of holes. These holes correspond in location to the contacts on the second surface of the printed wiring board. A plurality of connectors are positioned in the pattern of holes in the second interposer for electrically coupling the electrical contacts on the second surface of the printed circuit board to the electrical contacts on the surface of the second area array device. At least some of the electrical contacts on one or both of the generally planar surfaces of the printed wiring board comprise contact pads that are offset from the corresponding vias in the printed wiring board to which the contact pads are electrically connected. At least some of the connectors in the holes of said interposers have a first end soldered or pasted to contact pads on a surface of the printed wiring board, while the second end is pressed against the electrical contacts on the generally planar surface of the corresponding area array device without soldering or pasting. The invention also contemplates that the second interposer may separate the second surface of the printed wiring board a fixed minimum distance of about 3 mm from the surface of the second area array device.